The process of capturing images includes forming a visible image of a subject on a photosensitive surface by introducing light or other forms of radiation thereto. Image capturing devices are widely used for videography, photography, infrared photography, ultraviolet photography, stereoscopic photography, microphotography, and thermography. Such devices generally include video cameras, film cameras, and digital cameras.
Cameras basically include a light-tight body having an image capturing medium at a back end thereof, a shutter mechanism in front of the image capturing medium, an aperture in front of the shutter, and a lens disposed oppositely of the image capturing medium at a front end of the camera. The lens focuses light from a photographic subject through the aperture and shutter and onto the image-capturing medium to form an image of the subject thereon. The shutter and aperture together control exposure of the image-capturing medium. The shutter controls the length of time the image-capturing medium is exposed to light from the subject and the aperture is adjustable in size to control the amount of light from the subject that impinges on the image-capturing medium. Other camera features typically include a viewfinder to show the photographic subject, flash units to illuminate the photographic subject, and exposure meters to measure light.
Digital cameras are increasingly popular and technology advances are rapidly resulting in increased performance capability. A digital camera captures a subject, scene, or view in elemental portions and generates an electronic signal that is representative of the subject, scene, or view. Unlike conventional film cameras that use a photoresponsive film as the image capturing medium, digital cameras typically use electronic photosensors such as one or more charge coupled device (CCD) chips. The CCD chips are configured to receive light reflecting from the photographic subject and to convert the reflected light into an electronic signal. A CCD chip includes an array of very fine picture elements or “pixels” arranged in horizontal rows and vertical columns.
Upon exposure to imaging light from a subject, the CCD chips collect an array of discrete light energies or photon charges that correspond to or map the photographic subject column-by-column, row-by-row, and pixel-by-pixel such that a photon charge representation of the subject is seized. The CCD uses off-chip electronic circuits to process the photon charges and convert them into useful digital signals that can be stored in electronic memory either on or off-camera. Thus, digital cameras provide highly convenient features such as instant picture display and electronic storage format among many others, but provide image quality that is not as capable of handling low-light conditions as that of conventional film cameras.
Image resolution in digital cameras continues to improve as CCD chips with finer pixel sizes are developed that enable finer elemental representations of a photographic subject. Unfortunately, finer pixel size tends to adversely affect the already compromised low-light performance of a digital camera. More particularly, the light sensitivity or light gathering ability of a CCD decreases with reductions in pixel size over a given surface area of the CCD. In other words, for a CCD of a given surface area, the light gathering ability of the CCD decreases with increases in pixel count. This is because the CCD tends to lose optical sensing surface area to accommodate the width of borderlines between adjacent pixels. The result is increased image resolution via finer parsing of the view, but at a cost of reduced light gathering ability and hence reduced low-light level performance.
Low-light performance in digital cameras has typically been adjusted by varying the typical mechanical exposure settings of shutter speed, and aperture size, and also by adjusting the electronic setting of CCD output gain. One problem in varying shutter speed is a blurred image caused by relative movement between the camera and photographic subject while the shutter is open for a relatively long period of time to let more light into the camera. Also, a problem in varying gain is that increases in gain result in increases in noise and corresponding decreases in image quality. Digital cameras combine gain and exposure control into an automatic image enhancement feature. Unfortunately, such features are still susceptible to blur and noise problems and do not provide a user with readily accessible independent and dynamic range control of the resolution and light gathering ability of the image capturing medium.